1. Field of the Invention
The present invention relates to a micro power cell capable of piling up a disk type microfluidic-chip to generate an electric energy by a streaming potential of a fluid flowing through a multi micro channel of the chip.
2. Description of the Prior Art
When fluid flows through a charged channel by pressure gradient, the density of counter-ions whose charge is opposite to that of the wall surface becomes higher near the surface of the channel wall, resulting in an establishment of the electric double layer (so-called Debye layer). Counter-ions in the electric double layer generate the streaming current by the flow to a direction of the pressure gradient. Due to the potential difference caused by distribution gradient between the co-ions and the counter-ions, the streaming potential generates between upstream and downstream of the channel. Since counter-ions are accumulated in the downstream, the counter-ions move to the opposite direction of the pressure gradient (i.e., the opposite direction of the fluids flow) by the back diffusion effect, and thus conduction currents are induced. Note that a summation of the streaming current and the conduction current is zero at steady state, meaning the conservation of net current in the channel.
When the electrolyte solution prepared with arbitrary ionic concentration (i.e., the ionic strength) flows through the straight cylindrical channel having a well-defined circular cross-section, the relationship between the zeta potential ζ and the streaming potential E is given by equation 1 referred to as Helmholtz-Smoluchowski (H-S) equation.
                                          Δ            ⁢                                                  ⁢            E                                Δ            ⁢                                                  ⁢            p                          =                                            ɛ              o                        ⁢                          ɛ              r                        ⁢            ζ                                η            ⁢                                                  ⁢                          λ              o                                                          [                  Equation          ⁢                                          ⁢          1                ]            
wherein,
∈ denotes the potential induced by the electrokinetic effect, i.e., the streaming potential,
Δp denotes the applied pressure difference exerted between both ends of the channel,
∈o denotes the dielectric constant at vacuum or vacuum permittivity,
∈r denotes the relative permittivity of the electrolyte solution,
λo denotes the electric conductivity of the electrolyte solution, and
η denotes the viscosity of the electrolyte solution.
Many researchers have tried to understand the electrokinetic phenomena, and to analyze the fluid flowing in the microchannel with diameter less than several hundreds micrometers and the streaming potential according to it.
Rice and Whitehead addressed a correction factor from the analytic solution of Poisson-Boltzmann (P-B) equation that should be applied to the case of using the H-S equation (i.e., equation 1) when the surface potential is low in the paper [C. L. Rice, R. Whitehead, “Electrokinetic flow in a narrow cylindrical capillary”, J. Phys. Chem., 69, 4017-4024, 1965]. Levine et al. presented an analytic solution of the P-B equation to monovalent symmetric electrolyte solution with same mobilities for full range of the surface potential in the paper [S. Levine, J. R. Marriott, G. Neale, N. Epstein, “Theory of electrokinetic flow in fine cylindrical capillaries at high zeta-potentials”, J. Colloid Interface Sci., 52, 136-149, 1975].
The streaming potential, which is an opposite mechanism to the electro-osmosis, is one of the electrokinetic phenomena. The streaming potential has been considered as a useful method for determining the unknown zeta potential, corresponding to the surface potential of charged material. From studies disclosed in Causserand et al.'s paper [C. Causserand, M. Nyström, P. Aimar, “Study of streaming potentials of clean and fouled ultrafiltration membranes”, J. Membr. Sci. 88, 211-222, 1994] and Szymczyk et al.'s paper [A. Szymczyk, B. Aoubiza, P. Fievet, J. Pagetti, “Electrokinetic phenomena in homogeneous cylindrical pores”, J. Colloid Interface Sci. 216, 285-296, 1999], it can be known that the measurement of electrokinetic streaming potential effectively contributes to the charge characterization of the pore and the surface of porous material.
As disclosed in Sung et al.'s paper [J. H. Sung, M.-S. Chun, H. J. Choi, “On the behavior of electrokinetic streaming potential during protein filtration with fully and partially retentive nanopores”, J. Colloid Interface Sci. 264, 195-202, 2003], or Chun et al.'s US patent [M.-S. Chun, J.-J. Kim, S.-Y. Lee, “Equipment and method of local streaming potential measurement for monitoring the process of membrane fouling in hollow-fiber membrane filtration”, U.S. Pat. No. 6,277,099 B2, 2004], important information about the colloidal particle deposition onto the surface of the porous membrane can be obtained by monitoring the dynamic behavior of electrokinetic streaming potentials with time progress.
The physical meaning of above equation 1 is that when the electrolyte solution flows in a charged channel with a pressure gradient Δp, the potential difference ΔE occurs between the ends of the channel. Accordingly, connecting the external circuit applied with an appropriate resistance provides the electrical energy encountered by electric current and potential.
In recent years, studies were explored on whether a streaming potential occurring from the electrokinetic principle can be used as an energy source. D. Y. kwok, et al. reported that when tap water was allowed to flow in a porous glass filter having a pore size of 10˜16 μm (diameter: 2 cm, thickness: 3 mm, Schott-Duran, Mainz) by the pressure difference, a maximum streaming potential resulting from the flow was 10V and a maximum electric current was several μA [J. Yang, F. L. Larry, W. Kostiuk, D. Y. Kwok, “Electro-kinetic microchannel battery by means of electro-kinetic and microfluidic phenomena”, J. Micromech. Microeng., 13, 963-970, 2003].
Olthuis et al. reported the streaming potential and energy resulting from a change of an external resistance with regard to the same filter having a pore size of 1.0˜1.6 μm [W. Olthuis, B. Schippers, J. Eijkel, A. van den Berg, “Energy from streaming current and potential”, Sens. Actuators B, 111-112, 385-389, 2005].
In addition, Chun et al. established a theoretical model of an electrokinetic flow field and carried out the numerical computations to build theoretical origins capable of predicting a streaming potential in a multi-channel circuit [M.-S. Chun, T. S. Lee, N. W. Choi, “Microfluidic Analysis on Electrokinetic Streaming Potential by Microflows of Monovalent Electrolyte Solution”, J. Micromech. Microeng., 15, 710-719, 2005].
Until now, however, it has not been developed a power cell capable of commercializing the streaming potential.
Both the micro-electromechanical system (MEMS) process and micromachining technologies allow us to fabricate microchannels with desired channel dimension. Using the lab-on-a-chip technique based on these technologies, the micro total analysis system (μ-TAS) as well as the high throughput system (HTS) can be realized.
After the middle of 1990s, micromachining technology for the disposable plastic materials that are easily replicable in mass has been developed. The master mold for replication can be made by the traditional silicon-based micromachining technology. Like the study of Jo et al. [B.-H. Jo, L. M. van Lerberghe, K. M. Motsegood, D. J. Beebe, “Three-dimensional micro-channel fabrication in polydimethylsiloxane (PDMS) elastomer”, J. Microelectromech. Sys., 9(1), 76-81, 2000], a method using photoresist SU8 and polydimethylsiloxane (PDMS) is widely known as a simple and low-cost technology among the technologies using organic polymers.
To make the microchannel, a master mold patterning the channel shape is formed on the silicon wafer. Liquid PDMS prepolymer is poured over the mold, and then cure it. Subsequently, the PDMS replica is peeled from the master, and cut into a proper size. Then, the replica is sealed to either glass coverslip or PDMS substrate to enclose the channels.
As disclosed in the study of McDonald and Whitesides [J. C. McDonald, G. M. Whitesides, “Poly(dimethylsiloxane) as a material for fabricating microfluidic devices”, Acc. Chem. Res., 35(7), 491-498, 2002], the above method is easier in process and lower in cost than the conventional method of etching the glass or silicon wafer in view of mass production of microfluidic-chip.
Especially, since PDMS is indeed inert and transparent in the range of visible or near UV lights, it has merits as a material for microfluidic devices.